She especially stressed youngsters should avoid handling strange objects ahead of the start of the school holidays.

Dame Sally said: ‘I want to emphasise to everyone in the Salisbury and Amesbury area that nobody, adult or child, should pick up any foreign object which could contain liquid or gel, in the interests of their own safety.

‘This, in practice, means do not pick up containers, syringes, needles, cosmetics or similar objects, made of materials such as metal, plastic or glass.

It is believed that Dawn Sturgess, 44, who died from Novichok poisoning, and her partner, Charlie Rowley, 45, who is critically ill, somehow handled a vessel that contained the deadly substance.

The vessel has not yet been recovered.

There is no evidence to suggest they went to any of the areas that had been decontaminated following the attempted murders of Sergei Skripal and his daughter Yulia, who were also exposed to the nerve agent.

Dame Sally added: ‘Life continues in Salisbury and Amesbury and I want to be clear that the overall threat to the public is unchanged and remains low.